A hot runner system is a frequently employed assembly of heated components used in plastic injection molds to inject molten plastic into the cavities of a mold. The heated components comprise a plurality of heated injection tubes, each of which carries molten plastic to a cavity within the mold. In all hot runner systems a gating mechanism is required to open and close nozzles at the end of each injection tube. Most gating mechanisms employ a valve at the end of each injection nozzle. Each valve has a valve plug which is disposed at the end of a valve stem. Each valve stem is axially disposed within each injection tube. Each valve stem can axially travel back and forth within its injection tube to alternatively position its valve plug between (i) an open position, wherein the valve plug seals the opening at the end of an injection nozzle, and (ii) a closed position, wherein the valve plug is disposed away from the opening at the end of the injection tube.
One common gating mechanism is termed a “valve gate” mechanism. In a valve gate mechanism a pneumatic piston or similar driver is used to actuate each individual valve stem. Another common gating mechanism is termed a “common plate” mechanism (sometimes referred to as a “synchro-plate” mechanism). In common plate mechanisms a single (common) plate simultaneously actuates the valve stems for all of the injection tubes. The common plate is reciprocated in attachment with all valve stems to simultaneously drive them back and forth within their injection tubes. The reciprocation of the common plate is driven by one or more pneumatic pistons, hydraulic pistons or servo motors.
It is considered highly desirable to provide all gating mechanism with a mechanical shut off feature to allow the operator to shut off individual injection nozzles while continuing molding with the other nozzles. Providing a valve gate mechanism with a mechanical shut off feature is simply and easily accomplish by the use of sliding plates, each of which can be slid behind one of the hydraulic pistons used to drive a valve stem. Each sliding plate is situated such that it can be slid to a position where it blocks the travel of its hydraulic piston when the valve plug is in its closed position.
It is much more difficult with common plate actuators to provide a mechanical shut off feature, since the common plate reciprocates all of the valve stems. Accordingly, there is a need for an actuation apparatus in a common plate hot runner system which the operator can easily and conveniently shut down one of the injection tubes without affecting the operation of the remaining injection tubes.